1. Field of the Invention
This invention relates to a self-cleaning plate shaped-device for removing foreign substances from a plate member.
2. Description of the Related Art
Japanese laid-open patent publication No. 59,8548 published on Jan. 17, 1984 discloses a conventional cleaning device (see FIG. 16). In this device, a plate member 1 is provided as a rear-view mirror for an automobile. A piezo-electric vibrator 2 is fixed to the plate member 1. The vibrator 2 is substantially the same size as the plate member 1. An oscillator 3 is electrically connected to the vibrator 2 in order to supply A.C. power thereto. When the plate member 1 is oscillated by the vibrator 2, water drops on the plate member 1 are removed therefrom as they are caused to drip down and off the plate member and because they are atomized into a spray.
The size of the vibrator 2 should be substantially the same size as the plate member 1 in order to remove the water drops from the whole plate member 1. When the vibrator 2 is increased in size to be larger than the plate member, the vibrator 2 cannot be vibrated uniformly and localized vibration may be generated. Due to this localized vibration, the A.C. power is consumed inefficiently.
Japanese Utility Model laid-open patent publication No. 62-191550 published on Dec. 5, 1987 discloses another conventional cleaning device (see FIG. 17). In this device, the vibrator 2' is fixed substantially center of the plate member 1. The vibrator 2' is of smaller size than the plate member 1 and expands and contracts in a thickness direction thereof, i.e., in the direction of the arrow A.
In this device, the vibrator 2' can be vibrated uniformly, and the A.C. power may be conserved because the vibrator 2' is smaller than the plate member.
However, the thickness of the vibrator 2' cannot be reduced because the resonant frequency of the vibrator 2' increases according to reduction of the thickness thereof. If the resonant frequency of the vibrator 20 increases, the plate member 1 receives vibrations having a high frequency. At this time, a longitudinal wave may be generated on the entire plate member 1; however, the longitudinal wave is inefficiently transferred to the whole plate member 1. Accordingly, a strong vibration may be generated only on a portion of the plate member where the vibrator 2' is fixed, but only a weak vibration is generated on other portions of the plate member 1. As a result, the water drops off the plate at the portion near the vibrator 2', but the water remains at the other portions which are displaced from the vibrator 2'.